How can I concisely write a lot of explicit function template instantiations?

Question

I'm writing a C++ library that contains a lot of function templates I want to explicitly instantiate and export for several type parameters. In my particular case, I have a lot of numeric function templates that I want to separately instantiate and compile for float, double, and long double. They look something like this:

template <typename T>
T calculate_a(T x) { ... }

template <typename T>
T calculate_b(T x, T y) { ... }

// ...

If I have M function templates and N underlying types, then I have M*N explicit instantiations to type out. Is it possible to write these instantiations more concisely?

My current solution is to use a preprocessor macro that performs all instantiations for a given type:

#define EXPLICITLY_INSTANTIATE(T) \
    template T calculate_a<T>(T x); \
    template T calculate_b<T>(T x, T y); \
    // ...

EXPLICITLY_INSTANTIATE(float);
EXPLICITLY_INSTANTIATE(double);
EXPLICITLY_INSTANTIATE(long double);

However, this is suboptimal because it requires me to separately maintain another copy of each function template's signature. Also, if I want to do this in multiple translation units, then I need to separately maintain a list of underlying types in each. (Suppose that C++2a adds a long long double type that I want to support; I'll have to add EXPLICITLY_INSTANTIATE(long long double); to every file.)

Another possible approach is to gather up all of my functions into a (static-only) template class:

template <typename T>
class calculate {
    T a(T x) { ... }
    T b(T x, T y) { ... }
};

template class calculate<float>;
template class calculate<double>;
template class calculate<long double>;

This solves the first problem of separately maintaining two copies of each signature, but requires me to change each call of calculate_a into calculate::a<T>. It doesn't address the second problem.

Answer 1

You can avoid repeating function signatures by instantiating templates via taking their addresses:

// forward declarations in a header file
template<typename T>
T square(T num);

template<typename T>
T add(T left, T right);

// implementations and instantiations in a single implementation file
template<typename T>
T square(T num) {
    return num * num;
}

template<typename T>
T add(T left, T right) {
    return left + right;
}

// instantiations for specific types
#include <tuple>

template<typename... Ts>
auto instantiate() {
    static auto funcs = std::tuple_cat(std::make_tuple(
        add<Ts>,
        square<Ts>
    )...);

    return &funcs;
}

template auto instantiate<int, double>();

The overhead here is a single array of pointers to all instantiated functions, example on godbolt.

Answer 2

This is what X Macros are made for. It works quite simply.

You have a file that contains all of the types you want to apply this to. Let's call it "type_list.inc". It would look like this:

X(float)
X(double)
X(long double)

When you want to perform some operation over that list of types, you #include the file, but around the point of inclusion, you #define the macro X to do the operation you want to perform:

#define X(T) \
    template T calculate_a<T>(T x); \
    template T calculate_b<T>(T x, T y); \
    // ...
#include "type_list.inc"
#undef X

You still have to maintain two sets of function prototypes. But you only need to maintain one list of types.